1. Field of the Invention
The present invention relates to radiation systems.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
Typically, lithography systems use lasers as radiation sources to produce an illumination beam. Lasers typically produce polarized light, e.g., linearly, circularly, or elliptically polarized light. A problem with using polarized light as in an exposure process is that different polarization directions interact differently with respect to different types of coatings on optical elements within a lithography system. This can result in varying critical dimensions (CDs) in features formed on the substrate. Also, in masked-based systems, CDs of features formed on the substrate can vary because different directions of polarized light interact differently with diffraction patterns on a mask since the diffraction patterns can be polarization direction dependent. Thus, as a result of coatings or diffraction patterns, polarized light can result in varying CDs in features formed on the substrate.
This has been somewhat alleviated by using a compensation element made from birefringence material shaped into a wedge. In this case, the position on the wedge where a respective portion of the light enters will control what polarization direction the respective portion of the light will exhibit at an exit of the wedge. However, this has not been used to completely correct the imaging problems at the substrate.
Therefore, what is needed is a system and method that produces an unpolarized beam of radiation in order to substantially alleviate critical dimension variations in features formed on a substrate.